In a semiconductor device, such as a light-emitting diode (LED) or laser diode (LD), a layer of an n-type semiconductor material and a layer of a p-type semiconductor material are sequentially stacked on a suitable substrate, such as gallium arsenide, indium phosphide, silicon carbide or sapphire. In general, the n-type semiconductor layer is formed over the substrate layer, and the p-type semiconductor layer is formed above the n-type layer. In a typical example of an LED, a p-side electrode layer is formed on the p-type semiconductor layer, through which light is observed. In this device, the p-electrode layer must be very thin in order to be light transmissive, and thus, it is difficult to attach a bonding wire directly to it. Accordingly, a bonding pad is commonly used to attach the bonding wire, such as a gold wire, to the p-side electrode.
The bonding pad and electrode include one or more metal layers. Examples of the metal materials include nickel, copper, palladium, platinum and aluminum. In general, the top layer of the bonding pad is a gold layer because it can form a good bond with a gold wire. These metal layers are annealed at high temperature, for example, above 400° C. to obtain highly desirable electrical properties. During the high-temperature annealing process, metals in the bonding pad or in the electrode under the bonding pad, such as nickel, copper, palladium, platinum and aluminum, diffuse to the top surface of the bonding pad. In particular, nickel, copper, palladium, platinum and aluminum have a large diffusion coefficient through the grain boundaries of a top layer, for example, a gold layer, and diffusion of these materials can occur at temperatures as low as 100° C. The metal materials that diffuse to the top surface of the bonding pad are now subject to oxidation through various heat and/or chemical treatments associated with the fabrication of the semiconductor devices, rendering the surface of the bonding pad less suitable for subsequent wire bonding.
Similarly, diffusion of metal materials can also occur in an n-side electrode over an n-type semiconductor layer and a bonding pad for the n-side electrode or for the n-type semiconductor material during, for example, high temperature annealing, thereby weakening the bonding strength between a bonding wire and the bonding pad.
Therefore, there is a need for developing methods for reducing or preventing diffusion of oxidizable metal materials to the top surfaces of bonding pads.